The invention concerns the crowns of tires and, notably, the rubber decouplings placed between the reinforcing plies of those crowns.
The crowns of tires usually comprise a carcass reinforcement, a belt reinforcement with usually at least two superposed reinforcing plies formed by cords parallel in each ply and criss-crossed from one ply to the other and a tread.
The crowns of tires are in contact with the road and must transmit to the wheel, by means of the sidewalls and beads, the transverse stresses necessary to steer the vehicles. For the road performance of vehicles to be satisfactory, it is necessary for the crowns to be very rigid relative, for example, to the sidewalls. A permanent objective is to try to obtain such high rigidity as simply and economically as possible.
It is well known that the rubber decouplings of belt reinforcing cords contribute to obtaining high rigidity. That is why those rubber decouplings usually have a high modulus of elasticity. On the other hand, the rubber decouplings in contact with the cords of carcass reinforcements usually have a low modulus of elasticity because they must withstand without damage the high deformations they undergo in the sidewalls of the tires.
On the other hand, a great deal of research is being conducted to reduce the gasoline consumption of road vehicles. For that purpose, emphasis is on the design of tires having a very low rolling resistance, while maintaining the other properties of wear, adherence, performance, etc., as economically as possible.
The object of the invention is a tire whose crown structure is improved in order to facilitate its manufacturing process and thus make it more economical, as well as to improve its quality and its performances.
The object of the invention, according to a first variant, is also an embodiment intended mainly to lower the resistance to rolling and, according to a second variant, an embodiment intended to improve the rigidity of the crown.
In what follows, xe2x80x9ccordxe2x80x9d is understood to mean monofilaments as well as multifilaments, or assemblages like cables, yarns or even any type of equivalent assemblage, whatever the material and treatment of those cords, such as surface treatment or coating, notably, of rubber having undergone the start of vulcanization or pre-sizing to promote adhesion on the rubber.
The xe2x80x9crubber decoupling layerxe2x80x9d between two reinforcing plies is understood to mean a rubbery compound in contact with the reinforcing plies of at least one of the two plies, adhering to the latter and filling the interstices between adjacent cords. In current industrial practice, the reinforcing plies are made by application of calendered plies; consequently, between two given reinforcing plies, two rubber decoupling layers are used, each being in contact with the cords of one of the two plies and having the same composition for the different zones of the tire.
xe2x80x9cContactxe2x80x9d between a cord and a rubber decoupling layer is understood to mean that at least one part of the outer circumference of the cord is in close contact with the rubbery compound constituting the rubber decoupling. If the cord contains a covering or coating, the term contact means that the outer circumference of that covering or coating is in close contact with the rubbery compound constituting the rubber bonding.
xe2x80x9cModulus of elasticityxe2x80x9d of a rubbery compound is understood to mean a secant extension modulus obtained on a uniaxial extension deformation in the order of 10% after three cycles of accommodation and at ambient temperature.
Upon a sinusoidal stress of a rubbery compound, for example, with deformation applied, xcex5*=xcex50ejxcfx89t, the steady state response of that compound is also sinusoidal and dephased by an angle xcex4, "sgr"*="sgr"0ej(xcfx89t+xcex4). A complex modulus G*="sgr"*/xcex5*="sgr"0/xcex50ejxcex4=Gxe2x80x2+jGxe2x80x3 is defined, with "sgr" the stress in MPa. Gxe2x80x2 is called xe2x80x9cdynamic modulusxe2x80x9d and Gxe2x80x3 is called dynamic modulus of loss. The tan xcex4=Gxe2x80x3/Gxe2x80x2 ratio is called damping ratio. The measurements are made on alternate shear stress at a frequency of 10 Hz, at a temperature of 60xc2x0 C. and at a peak-to-peak dynamic deformation of 10%.
The tire according to the invention comprises a crown, two sidewalls and two beads, a carcass reinforcement anchored in the two beads and a belt reinforcement, that belt reinforcement comprising at least two superposed reinforcing plies formed by cords parallel in each ply and criss-crossed from one ply to the other by forming angles (xcex1, xcex2) with the circumferential direction ranging between 10xc2x0 and 70xc2x0. That tire is characterized in that, between the two superposed reinforcing plies, at least two rubber decoupling layers of different mechanical properties are placed axially adjacent, and in that each of said two rubber decoupling layers is in contact with the cords of said two superposed reinforcing plies.
That tire has the advantage of having only one single rubber decoupling layer between the two crown reinforcing plies in any circumferential section (outside of the zones of transition between the two adjacent rubber decoupling layers). This layer can have different properties in different circumferential sections, for example, in the center and on the edges of the plies. That makes it possible to adapt the nature and the properties of the single layer to the objective of the tire designer.
The rubber decoupling layers are preferably made by a spiral winding of a rubbery compound section directly on the cords of the radially inner reinforcing ply. That direct application of rubber decoupling layers on the cords of the reinforcing plies simplifies manufacture of the tire.
A first decoupling layer is placed between the center part of the two superposed reinforcing plies and a second decoupling layer is placed on at least one side of the first layer and extends at least as far as the corresponding lateral ends of the two superposed reinforcing plies. In a particular embodiment, a second decoupling layer extends axially more than 3 mm beyond a lateral end of the cords of a reinforcing ply.
In a first embodiment, the ratio between the moduli of elasticity of the second layer and first layer ranges between 0.05 and 0.8 and preferably between 0.4 and 0.6.
In that embodiment, the modulus of elasticity of the rubber decouplings between the two reinforcing plies consisting of crossed cords is markedly less in at least one lateral zone of the tire crown than that of the rubber decouplings in the center zone of the crown. This has the advantage of making substantial gains in running resistance possible by maintaining the drift thrust and rolling lifetime properties at totally acceptable levels.
The second rubber decoupling layer preferably has a damping ratio tan xcex4 below 0.08. Such a layer is very easily hysteretic and markedly contributes to the gain in running resistance of the tire concerned.
It is also possible, in order to preserve or improve the drift thrust and lifetime properties, to add, on the same side as the second rubber decoupling layer, an additional reinforcing ply consisting of cords oriented in the circumferential direction and extending axially roughly like the second rubber decoupling layer, for example having substantially the same inner and outer axial extension as the second rubber decoupling layer. That additional reinforcing ply can be placed radially outside or inside the two superposed reinforcing plies or between those two plies.
This first embodiment of a tire is suited, in particular, to tires of H/W aspect ratio higher than 0.55
According to a second embodiment, the ratio between the moduli of elasticity of the second rubber decoupling layer and first layer ranges between 1.2 and 20 and preferably between 1.5 and 10.
In that second embodiment, it is the first rubber decoupling layer, arranged in the center zone of the two reinforcing plies, which has a low modulus of elasticity, less than that of the rubber decoupling layer arranged in the lateral zone of the tire crown.
That second embodiment also has the advantage of making it possible to reduce the running resistance without impairing the drift thrust and lifetime properties. That second embodiment is of particular value with tires of aspect ratio below 0.55.
According to another embodiment of the invention, the two reinforcing plies between which two rubber decoupling layers of different mechanical properties are axially placed are a crown reinforcing ply and a carcass reinforcing ply. That embodiment is particularly advantageous when the belt reinforcement consists of a reinforcing ply whose cords are oriented relative to the circumferential direction at an angle xcex1, ranging between 10xc2x0 and 70xc2x0 and of a reinforcing ply formed by parallel cords oriented roughly circumferentially.